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"Animation of carbon atoms, shown as the Bohr model. At the centre is the tightly packed nucleus, composed of protons (orange) and neutrons (yellow). Surrounding the nucleus are the electrons (blue), arranged in a series of discrete energy levels, or orbitals. The protons carry a positive charge, which is balanced by the negative charge of the same number of electrons. Each orbital can only hold a set number of electrons, with two in the inner shell, and four in the outer one in a carbon atom. The arrangement of electrons in orbitals is responsible for an atom's chemical properties."